Thursday, August 30, 2012

An important new paper published today in Global and Planetary Change finds that changes in CO2 follow rather than lead global air surface temperature and that "CO2 released from use of fossil fuels have little influence on the observed changes in the amount of atmospheric CO2" The paper finds the "overall global temperature change sequence of events appears to be from 1) the ocean surface to 2) the land surface to 3) the lower troposphere," in other words, the opposite of claims by global warming alarmists that CO2 in the atmosphere drives land and ocean temperatures. Instead, just as in the ice cores, CO2 levels are found to be a lagging effect of ocean warming, not significantly related to man-made emissions, and not the driver of warming. Prior research has shown infrared radiation from greenhouse gases is incapable of warming the oceans, only shortwave radiation from the Sun is capable of penetrating and heating the oceans and thereby driving global surface temperatures. The highlights of the paper are:► The overall global temperature change sequence of events appears to be from 1) the ocean surface to 2) the land surface to 3) the lower troposphere. ► Changes in global atmospheric CO2 are lagging about 11–12 months behind changes in global sea surface temperature. ► Changes in global atmospheric CO2 are lagging 9.5-10 months behind changes in global air surface temperature. ► Changes in global atmospheric CO2 are lagging about 9 months behind changes in global lower troposphere temperature. ► Changes in ocean temperatures appear to explain a substantial part of the observed changes in atmospheric CO2 since January 1980. ► CO2 released from use of fossil fuels have little influence on the observed changes in the amount of atmospheric CO2, and changes in atmospheric CO2 are not tracking changes in human emissions.

Figure 1: Annual temperature change of global ocean temperatures, global surface temperature and atmospheric CO2 December 1981 - December 2011.(Blue is the sea surface, red is the global surface temperature, green is CO2 level in the atmosphere).We see that the change in ocean temperatures (blue) occur systematically before changes in CO2 (green).

Abstract

Using data series on atmospheric carbon dioxide and global temperatures we investigate the phase relation (leads/lags) between these for the period January 1980 to December 2011. Ice cores show atmospheric CO2 variations to lag behind atmospheric temperature changes on a century to millennium scale, but modern temperature is expected to lag changes in atmospheric CO2, as the atmospheric temperature increase since about 1975 generally is assumed to be caused by the modern increase in CO2. In our analysis we use eight well-known datasets; 1) globally averaged well-mixed marine boundary layer CO2 data, 2) HadCRUT3 surface air temperature data, 3) GISS surface air temperature data, 4) NCDC surface air temperature data, 5) HadSST2 sea surface data, 6) UAH lower troposphere temperature data series, 7) CDIAC data on release of anthropogene CO2, and 8) GWP data on volcanic eruptions. Annual cycles are present in all datasets except 7) and 8), and to remove the influence of these we analyze 12-month averaged data. We find a high degree of co-variation between all data series except 7) and 8), but with changes in CO2 always lagging changes in temperature. The maximum positive correlation between CO2 and temperature is found for CO2 lagging 11–12 months in relation to global sea surface temperature, 9.5-10 months to global surface air temperature, and about 9 months to global lower troposphere temperature. The correlation between changes in ocean temperatures and atmospheric CO2 is high, but do not explain all observed changes.

Highlights

► The overall global temperature change sequence of events appears to be from 1) the ocean surface to 2) the land surface to 3) the lower troposphere. ► Changes in global atmospheric CO2 are lagging about 11–12 months behind changes in global sea surface temperature. ► Changes in global atmospheric CO2 are lagging 9.5-10 months behind changes in global air surface temperature. ► Changes in global atmospheric CO2 are lagging about 9 months behind changes in global lower troposphere temperature. ► Changes in ocean temperatures appear to explain a substantial part of the observed changes in atmospheric CO2 since January 1980. ► CO2 released from use of fossil fuels have little influence on the observed changes in the amount of atmospheric CO2, and changes in atmospheric CO2 are not tracking changes in human emissions.

A geoengineering paper published today in Environmental Research Letters proposes spending $8 billion per year to pump 5 million tons of sulfuric acid [H2SO4] into the high atmosphere to reflect sunlight, to allegedly save the planet from the non-existent problem of anthropogenic global warming. The authors propose a 20 kilometer pipe connected to a "new aircraft design," but note their scheme has a "large uncertainty" that either the pipe or aircraft could fail, thereby spewing tons of sulfuric acid directly onto Gaia.

The full paper is here and states the "albedo modification material" of choice is H2SO4, which is sulfuric acid.

Cost analysis of stratospheric albedo modification delivery systems

Letter

We perform engineering cost analyses of systems capable of delivering 1–5 million metric tonnes (Mt) of albedo modification material to altitudes of 18–30 km. The goal is to compare a range of delivery systems evaluated on a consistent cost basis. Cost estimates are developed with statistical cost estimating relationships based on historical costs of aerospace development programs and operations concepts using labor rates appropriate to the operations. We evaluate existing aircraft cost of acquisition and operations, perform in-depth new aircraft and airship design studies and cost analyses, and survey rockets, guns, and suspended gas and slurry pipes, comparing their costs to those of aircraft and airships. Annual costs for delivery systems based on new aircraft designs are estimated to be $1–3B to deliver 1 Mt to 20–30 km or $2–8B to deliver 5 Mt to the same altitude range. Costs for hybrid airships may be competitive, but their large surface area complicates operations in high altitude wind shear, and development costs are more uncertain than those for airplanes. Pipes suspended by floating platforms provide low recurring costs to pump a liquid or gas to altitudes as high as ~ 20 km, but the research, development, testing and evaluation costs of these systems are high and carry a large uncertainty; the pipe system's high operating pressures and tensile strength requirements bring the feasibility of this system into question. The costs for rockets and guns are significantly higher than those for other systems. We conclude that (a) the basic technological capability to deliver material to the stratosphere at million tonne per year rates exists today, (b) based on prior literature, a few million tonnes per year would be sufficient to alter radiative forcing by an amount roughly equivalent to the growth of anticipated greenhouse gas forcing over the next half century, and that (c) several different methods could possibly deliver this quantity for less than $8B per year. We do not address here the science of aerosols in the stratosphere, nor issues of risk, effectiveness or governance that will add to the costs of solar geoengineering.

Wednesday, August 29, 2012

Color-enhanced image of sea surface heights in the Gulf of Mexico, showing Hurricane Isaac's path through the Gulf and around its warmest waters. (Credit: LSU Earth Scan Laboratory/U. of Colorado CCAR/NASA-JPL/Caltech)

ScienceDaily (Aug. 29, 2012) — Seven years after the powerful Category 3 Hurricane Katrina caused widespread devastation along the Gulf Coast, a Category 1 Hurricane Isaac, with maximum sustained winds of 80 miles per hour (70 knots), made landfall Aug. 28 in southeast Louisiana. And one of the reasons why Isaac is not Katrina is the path it took across the Gulf of Mexico and the temperature of the ocean below, which helps to fuel hurricanes.

In 2005, Hurricane Katrina's maximum wind speeds increased dramatically as the storm passed over a warm ocean circulation feature called the Loop Current that is part of the Gulf Stream. The storm evolved quickly from a Category 3 to a Category 5 event on the Saffir-Simpson Hurricane Wind Scale in a matter of nine hours as it drew heat from the Loop Current. It subsequently dropped in intensity to a Category 3 storm at landfall.

Because the Loop Current and its eddies are warmer, and thus higher in surface elevation, than the surrounding waters, they are easily spotted by satellite altimeter instruments, such as those aboard the NASA/French Space Agency Jason 1 and Ocean Surface Topography Mission/Jason 2 satellites. Scientists use the latest satellite measurements of sea-surface height from these and other satellite altimeters to create maps showing the location, direction and speed of currents in the Gulf of Mexico.

This color-enhanced image of sea surface heights in the northeastern Gulf, produced using data from available satellite altimeters, including NASA's Jason-1 and Jason-2 satellites, shows Isaac's path through the Gulf. The storm skirted around the Loop Current, then caught the outer edge of a warm eddy before passing directly over a cold eddy. The storm's track away from the Gulf's warmest waters has helped to keep Isaac from intensifying rapidly, as Hurricanes Katrina and Rita did in 2005.

Warm eddies have high heat content and great potential to intensify hurricanes, whereas cold eddies have low heat content and may even cause hurricanes to weaken, as was the case with Hurricane Ivan in 2004.

A new geoengineering paper proposes to use the net negative feedback of clouds to lower sea surface temperatures in hurricane prone areas by "up to a few degrees" to allegedly reduce hurricane intensity by a category.

They are one of the most destructive forces of nature on Earth, but now environmental scientists are working to tame the hurricane. In a paper, published in Atmospheric Science Letters, the authors propose using cloud seeding to decrease sea surface temperatures where hurricanes form. Theoretically, the team claims the technique could reduce hurricane intensity by a category.

The team focused on the relationship between sea surface temperature and the energy associated with the destructive potential of hurricanes. Rather than seeding storm clouds or hurricanes directly, the idea is to target marine stratocumulus clouds, which cover an estimated quarter of the world's oceans, to prevent hurricanes forming.

"Hurricanes derive their energy from the heat contained in the surface waters of the ocean," said Dr Alan Gadian from the University of Leeds. "If we are able to increase the amount of sunlight reflected by clouds above the hurricane development region then there will be less energy to feed the hurricanes."

Using a technique known as Marine Cloud Brightening (MCB), the authors propose that unmanned vehicles could spray tiny seawater droplets, a good fraction of which would rise into the clouds above, increasing their droplet numbers and thereby the cloud reflectivity and duration. In this way, more sunlight is bounced back into space, thereby reducing sea surface temperature.

The team's calculations, based on a climate ocean atmosphere coupling model (HadGEM1) suggest this could reduce the power of developing hurricanes by one category. Somewhat different cloud-seeding projects, designed to directly influence rainfall amounts, already exist around the world and were most famously used in China during the 2008 Beijing Olympics.

"Data shows that over the last three decades hurricane intensity has increased in the Northern Atlantic, the Indian and South-West Pacific Oceans," said Gadian. "We simulated the impact of seeding on these three areas, with particular focus on the Atlantic hurricane months of August, September and October."

The calculations show that when targeting clouds in identified hurricane development regions the technique could reduce an average sea surface temperature by up to a few degrees, greatly decreasing the amount of energy available to hurricane formation.

One potential drawback to the idea is the impact of cloud seeding on rainfall in neighbouring regions. The team noted concerns that seeding in the Atlantic could lead to a significant reduction of rainfall in the Amazon basin and elsewhere. However, if different patterns of seeding were used, such rainfall reductions were not found over land.

"Much more research is needed and we are clear that cloud seeding should not be deployed until we are sure there will be no adverse consequences regarding rainfall," concluded Gadian. "However if our calculations are correct, judicious seeding of maritime clouds could be invaluable for significantly reducing the destructive power of future hurricanes."

Abstract

This paper examines the potential to cool ocean surface waters in regions of hurricane genesis and early development. This would be achieved by seeding, with copious quantities of seawater cloud condensation nuclei (CCN), low-level maritime stratocumulus clouds covering these regions or those at the source of incoming currents. Higher cloud droplet density would increase these clouds' reflectivity to incoming sunlight, and possibly their longevity. This approach is therefore a more localized application of the marine cloud brightening (MCB) geoengineering technique promoting global cooling. By utilizing a climate ocean/atmosphere coupled model, HadGEM1, we demonstrate that—subject to the satisfactory resolution of defined but unresolved issues—judicious seeding of maritime stratocumulus clouds might significantly reduce sea surface temperatures (SSTs) in regions where hurricanes develop. Thus artificial seeding may reduce hurricane intensity; but how well the magnitude of this effect could be controlled is yet to be determined.

We also address the important question as to how MCB seeding may influence precipitation. GCM modelling indicates that the influence of seeding on undesirable rainfall reductions depends on its location and magnitude. Much more work on this topic is required.

Tuesday, August 28, 2012

A paper published today in the Journal of Geophysical Research asks the question, "Why does the temperature rise faster in the arid region of northwest China?" The runaway greenhouse theory alleges that warming from greenhouse gases will be amplified by increased evaporation and atmospheric water vapor. According to the theory, wet areas with the most atmospheric water vapor should warm faster than arid areas with less. However, observations from 1960-2010 show that the dry region of China warmed faster than the rest of China and the entire globe. The authors explain this apparent paradox as primarily due to the Siberian High, a natural atmospheric circulation. CO2 is well-mixed in the atmosphere and therefore cannot account for different rates of warming in different regions.The finding that arid regions warm faster and cool faster than wet regions around the globe was confirmed by physicist Clive Best, who examined 5600 weather stations in the global CRUTEM4 temperature and humidity database, finding that water vapor acts as a strong negative feedback rather than a positive feedback as alleged by the IPCC.

[Clive Best] "the IPCC argues that feedbacks from increased water evaporation will lead to enhanced warming. This is not observed in those regions most effected by water vapour. In fact the opposite seems to be the case implying negative feedback."

During 1960–2010, the air temperature in the arid region of northwest China had a significant rising trend (P < 0.001), at a rate of 0.343°C/decade, higher than the average of China (0.25°C/decade) and that of the entire globe (0.13°C/decade) for the same period. Based on the analysis of the data from 74 meteorological stations in the region for 1960–2010, we found that among the four seasons the temperature change of winter has been playing the most important role in the yearly change in this region. We also found that the winter temperature in this region has a strong association with the Siberian High (correlation coefficient: R = −0.715) and the greenhouse gas emission (R = 0.51), and between the two the former is stronger. We thus suggest that the weakening of the Siberian High during the 1980s to 1990s on top of the steady increasing of the greenhouse emission is the main reason for the higher rate of the temperature rise in the arid region of the northwest China.

A paper published in the Journal of Climate finds regional precipitation trends from 1977-2006 were related to natural variability of sea surface temperatures, not man-made greenhouse gases or aerosols. The finding contradicts claims by alarmists that mankind has "loaded the dice" for more droughts, floods, and extreme weather. In addition, the paper finds climate models are very poor at predicting both the intensity and patterns of regional rainfall, "especially a simulated increase in rainfall over the tropical Pacific and southeastern Australia that are opposite in sign to the actual drying in these areas."

NOAA/Earth System Research Laboratory, and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado

Abstract

In this study, the nature and causes for observed regional precipitation trends during 1977–2006 are diagnosed. It is found that major features of regional trends in annual precipitation during 1977–2006 are consistent with an atmospheric response to observed sea surface temperature (SST) variability. This includes drying over the eastern Pacific Ocean that extends into western portions of the Americas related to a cooling of eastern Pacific SSTs, and broad increases in rainfall over the tropical Eastern Hemisphere, including a Sahelian rainfall recovery and increased wetness over the Indo–West Pacific related to North Atlantic and Indo–West Pacific ocean warming. It is further determined that these relationships between SST and rainfall change are generally not symptomatic of human-induced emissions of greenhouse gases (GHGs) and aerosols. The intensity of regional trends simulated in climate models using observed time variability in greenhouse gases, tropospheric sulfate aerosol, and solar and volcanic aerosol forcing are appreciably weaker than those observed and also weaker than those simulated in atmospheric models using only observed SST forcing. The pattern of rainfall trends occurring in response to such external radiative forcing also departs significantly from observations, especially a simulated increase in rainfall over the tropical Pacific and southeastern Australia that are opposite in sign to the actual drying in these areas.

Additional experiments illustrate that the discrepancy between observed and GHG-forced rainfall changes during 1977–2006 results mostly from the differences between observed and externally forced SST trends. Only weak rainfall sensitivity is found to occur in response to the uniform distribution of SST warming that is induced by GHG and aerosol forcing, whereas the particular pattern of the observed SST change that includes an increased SST contrast between the east Pacific and the Indian Ocean, and strong regional warming of the North Atlantic Ocean, was a key driver of regional rainfall trends. The results of this attribution study on the causes for 1977–2006 regional rainfall changes are used to discuss prediction challenges including the likelihood that recent rainfall trends might persist.

Monday, August 27, 2012

A new paper published in Nature Geoscience finds "From about 50,000 to 11,000 years ago, the central Arctic Basin from 1,000 to 2,500 meters deep was ... 1–2°C warmer than modern Arctic Intermediate Water." This finding is particularly surprising because it occurred during the last major ice age.

Horizontal axis is thousands of years ago with modern temperatures at the left and 50,000 years ago at the right. Temperature proxy of the Intermediate Water Layer of the Arctic Ocean is shown in top graph with degrees C anomaly noted at the upper right vertical axis. Note this graph is on an inverse scale with warmer temps at the bottom and colder temps at the top.

In the Arctic Ocean, the cold and relatively fresh water
beneath the sea ice is separated from the underlying warmer
and saltier Atlantic Layer by a halocline. Ongoing sea ice
loss and warming in the Arctic Ocean have
demonstrated the instability of the halocline, with
implications for further sea ice loss. The stability of the
halocline through past climate variations is unclear.
Here we estimate intermediate water temperatures over the
past 50,000 years from the Mg/Ca and Sr/Ca values of
ostracods from 31 Arctic sediment cores. From about 50 to11 [thousand years] ago, the central Arctic Basin from 1,000 to 2,500m was occupied by a water mass we call Glacial Arctic Intermediate Water. This water mass was 1–2°C warmer than modern Arctic Intermediate Water,
with temperatures
peaking during or just before millennial-scale Heinrich cold
events and the Younger Dryas cold interval. We use
numerical modelling to show that the intermediate depth
warming could result from the expected decrease in the flux
of fresh water to the Arctic Ocean during glacial conditions,
which would cause the halocline to deepen and push the
warm Atlantic Layer into intermediate depths. Although not
modelled, the reduced formation of cold, deep waters due to
the exposure of the Arctic continental shelf could also
contribute to the intermediate depth warming.

Saturday, August 25, 2012

A paper published in Science finds summer Arctic Sea Ice extent during the Holocene Thermal Maximum 8,000 years ago was "less than half of the record low 2007 level." The paper finds a "general buildup of sea ice from ~ 6,000 years before the present" which reached a maximum during the Little Ice Age and "attained its present (year 2000) extent at 4,000 years before the present"

Horizontal axis is number of years before the present. Multiyear sea ice reached a minimum between ~8500-6000 years ago during the Holocene Thermal Maximum (HTM).

Excerpt: In general, our sea-ice record for North Greenland follows the Holocene climate development, with an early warm period followed by declining temperatures, which were punctuated by relatively warmer and colder intervals (17, 25). The reduction of the HTM sea ice in northern Greenland fits with the simulated ice distribution and surface temperature in orbitally forced ECHAM5/JSBACH/MPI-OM (EJM) and LOVECLIM general circulation climate model simulations (3, 4, 10). A tentative first approximation of the large-scale changes associated with the observed ice retreat north of Greenland can be obtained by selecting among the numerical experiments performed with the LOVECLIM model those that are the most similar to our observations [experiments E3 to E5 (3) and fig. S3]. In this exercise, our records would correspond in the model to an Arctic Ocean sea-ice cover in summer at 8 ky B.P. that was less than half of the record low 2007 level. The general buildup of sea ice from ~6 ky B.P. agrees with the LOVECLIM model, showing that summer sea-ice cover, which reached its Holocene maximum during the LIA, attained its present (~2000) extent at ~4 ky B.P. (fig. S3)

ABSTRACT

We present a sea-ice record from northern Greenland covering the past 10,000 years. Multiyear sea ice reached a minimum between ~8500 and 6000 years ago, when the limit of year-round sea ice at the coast of Greenland was located ~1000 kilometers to the north of its present position. The subsequent increase in multiyear sea ice culminated during the past 2500 years and is linked to an increase in ice export from the western Arctic and higher variability of ice-drift routes. When the ice was at its minimum in northern Greenland, it greatly increased at Ellesmere Island to the west. The lack of uniformity in past sea-ice changes, which is probably related to large-scale atmospheric anomalies such as the Arctic Oscillation, is not well reproduced in models. This needs to be further explored, as it is likely to have an impact on predictions of future sea-ice distribution.

Friday, August 24, 2012

A paper published today in the Journal of Applied Meteorology and Climatology proposes a new pie-in-the-sky geoengineering scheme to remove the harmless, trace [0.0393%], essential gas CO2 from the atmosphere using 446 deposition factories in Antarctica to produce CO2-laden snow. According to the authors, Antarctica is the ideal location for this scheme due to required temperatures of 133°K, equivalent to -140°C or -220°F.

A scientific plan is presented that proposes the construction of CO2 deposition plants in the Antarctic for removing CO2gas from the Earth's atmosphere. The Antarctic continent offers the best environment on Earth for CO2 deposition at 1 bar of pressure, and temperatures closest to that required for terrestrial air CO2 snow deposition,133°K. This plan consists of several components, including: (a) air chemistry and CO2 snow deposition, (b) the deposition plant and a closed-loop liquid nitrogen refrigeration cycle, (c) the mass storage landfill, (d) power plant requirements, (e) prevention of dry ice sublimation and (f) disposal (or use) of thermal waste. Calculations demonstrate that this project is worthy of consideration, whereby 446 deposition plants supported by 16 1200-MW wind farmscan remove 1 B tons (1012 kg) of CO2 annually (a reduction of 0.5 ppmv), which can be stored in an equivalent “landfill” volume of 2 km x 2 km x 160 m (insulated to prevent dry ice sublimation).

The individual deposition plant, with a 100m x 100m x 100m refrigeration chamber, would produce approximately 0.4m of CO2 snow per day. The solid CO2 would be excavated into a 380m x 380m x 10m insulated landfill, that would allow one year of storage amounting to 0.00224B tons of carbon. Demonstrated success of a prototype system in the Antarctic would be followed by a complete installation of all 446 plants for CO2 snow deposition and storage (amounting to 1B tons annually), with wind farms positioned in favorable coastal regions with katabatic wind currents.

Thursday, August 23, 2012

A new paper published in PNAS finds from 2 sediment temperature proxies that the upper 1,500 feet of the tropical Atlantic Ocean experienced abrupt warming of about 3-4C during each of at least two periods over the past 22,000 years due to a natural change in ocean oscillations [i.e. not greenhouse gases]. The paper also shows "the modern mean annual temperature" is about 0.5 to 2.5C colder than the end of the proxy records around 500 years before the present.

Graph and legend from the paper's supplemental information. The final segments of both temperature proxies in blue and red were added to connect the proxy temperatures to "the modern mean annual temperatures" The Younger Dryas [YD] was a period of natural global warming at a much faster rate than over the 20th century. Horizontal x axis is thousands of years before the present [ky BP].

Edited by James C. McWilliams, UCLA, Los Angeles, CA, and approved August 1, 2012 (received for review May 8, 2012)

Abstract

Both instrumental data analyses and coupled ocean-atmosphere models indicate that Atlantic meridional overturning circulation (AMOC) variability is tightly linked to abrupt tropical North Atlantic (TNA) climate change through both atmospheric and oceanic processes. Although a slowdown of AMOC results in an atmospheric-induced surface cooling in the entire TNA, the subsurface experiences an even larger warming because of rapid reorganizations of ocean circulation patterns at intermediate water depths. Here, we reconstruct high-resolution temperature records using oxygen isotope values and Mg/Ca ratios in both surface- and subthermocline-dwelling planktonic foraminifera from a sediment core located in the TNA over the last 22 ky. Our results show significant changes in the vertical thermal gradient of the upper water column, with the warmest subsurface temperatures of the last deglacial transition corresponding to the onset of the Younger Dryas. Furthermore, we present new analyses of a climate model simulation forced with freshwater discharge into the North Atlantic under Last Glacial Maximum forcings and boundary conditions that reveal a maximum subsurface warming in the vicinity of the core site and a vertical thermal gradient change at the onset of AMOC weakening, consistent with the reconstructed record. Together, our proxy reconstructions and modeling results provide convincing evidence for a subsurface oceanic teleconnection linking high-latitude North Atlantic climate to the tropical Atlantic during periods of reduced AMOC across the last deglacial transition.

ScienceDaily (Aug. 23, 2012) — A new record of past temperature change in the tropical Atlantic Ocean's subsurface provides clues as to why Earth's climate is so sensitive to ocean circulation patterns, according to climate scientists at Texas A&M University.

Geological oceanographer Matthew Schmidt and two of his graduate students teamed up with Ping Chang, a physical oceanographer and climate modeler, to help uncover an important climate connection between the tropics and the high latitude North Atlantic. Their new findings are in the current issue ofPNAS (Proceedings of the National Academy of Sciences).

The researchers used geochemical clues in fossils called foraminifera, tiny sea creatures with a hard shell, collected from a sediment core located off the northern coast of Venezuela, to generate a 22,000-year record of past ocean temperature and salinity changes in the upper 1,500 feet of water in the western tropical Atlantic. They also conducted global climate model simulations under the past climate condition to interpret this new observational record in the context of changes in the strength of the global ocean conveyor-belt circulation.

"What we found was that subsurface temperatures in the western tropical Atlantic rapidly warmed during cold periods in Earth's past," Schmidt explains.

"Together with our new modeling experiments, we think this is evidence that when the global conveyor slowed down during cold periods in the past, warm subsurface waters that are normally trapped in the subtropical North Atlantic flowed southward and rapidly warmed the deep tropics. When the tropics warmed, it altered climate patterns around the globe."

He notes that as an example, if ocean temperatures were to warm along the west coast of Africa, the monsoon rainfall in that region would be dramatically reduced, affecting millions of people living in sub-Saharan Africa. The researchers also point out that the southward flow of ocean heat during cold periods in the North Atlantic also causes the band of rainfall in the tropics known as the Intertropical Convergence Zone to migrate southward, resulting in much drier conditions in northern South American countries and a wetter South Atlantic.

"Evidence is mounting that the Earth's climate system has sensitive triggers that can cause abrupt and dramatic shifts in global climate," Schmidt said.

"What we found in our subsurface reconstruction was that the onset of warmer temperatures, thought to reflect the opening of this 'gateway' mechanism, occurred in less than a few centuries. It also tells us that it might be a good idea to monitor subsurface temperatures in the western tropical Atlantic to assess how the strength of the ocean conveyor might be changing over the next few decades as Earth's climate continues to warm."

"One way to prepare for future climate change is to increase our understanding of how it has operated in the recent past."